In recent years, an aqueous ink composition (may be referred to as an “aqueous ink” hereinafter) has taken the place of an organic solvent-based ink in a wide range of fields because of its safety and little environmental load, and particularly an odorless aqueous ink has become essential as an ink which can be used even in a closed space such as an office or the like.
The aqueous ink is applied to various printing methods such as an offset printing method, a gravure printing method, an ink jet recording method, and the like. In particular, the ink jet recording method is capable of on-demand printing and is thus currently often investigated for the purpose of replacing an electrophotographic (toner) method used in offices and the like.
The physical properties required for an aqueous ink using an ink jet recording method include both the color development (optical density) and rubfastness of a formed image. For this requirement, there is known a two-liquid reaction system using an image forming ink and a fixing ink (also referred to as a “treatment agent”) (for example, refer to Patent Literatures 1 to 3). However, in the case of the two-liquid reaction system, printing is actually performed in two steps using the image forming ink and the fixing ink, and thus it is difficult to realize high-speed printing at a level equal or higher than the electrophotographic (toner) method.
On the other hand, bio-nanofibers are produced by uniformly refining (forming nanofibers) cellulose or chitin/chitosan at a nano-level and, in an industrial application, bio-nanofibers are known to be added for the purpose of improving the drying properties and toughness of coating films by imparting thixotropic viscosity to aqueous emulsion compositions for coat bonding and filling (for example, refer to Patent Literature 4), and added to a pigment composition for the purpose of imparting good rheology characteristics (for example, refer to Patent Literature 5). However, there has not been known an example of use for the purpose of improving the optical density and rubfastness of an aqueous ink.
An aqueous ink for writing implements is known, in which an aqueous ink is mixed with modified cellulose nanofibers characterized by a complex structure in order to secure excellent dispersion stability even of a coloring agent with high specific gravity (for example, refer to Patent Literature 6). The modified cellulose nanofibers described in Patent Literature 6 have a predetermined cellulose crystal structure and are required to be subjected to reduction reaction to adjust a content of carboxyl groups and a total content of aldehyde groups and ketone groups after selective oxidation of hydroxyl groups at specified positions in a cellulose molecule. Therefore, there is the problem of high cost and a large chemical limit of ink composition design.
On the other hand, an aqueous ink for ink jet recording is known, in which rubfastness is improved by adding colloidal silica. For example, Patent Literature 4 discloses an ink composition for ink jet, which contains at least one water-soluble organic solvent, water, at least one coloring material, and at least one type of colloidal silica, wherein the content of a water-soluble organic solvent with a solubility parameter (SP value) of 27.5 or less among the water-soluble organic solvent is 40% by mass or more, and also discloses that an image with excellent rubfastness under higher-speed recording than usual recording and suppressed occurrence of blocking can be formed. However, Patent Literature 7 makes no mention about color development (optical density).